Vehicle steering system transmission

ABSTRACT

A vehicle steering system transmission comprising a housing, an input shaft journalled to the housing, an electric motor connected to the housing and coupled to the input shaft, an output shaft journalled to the housing, the input shaft and the output shaft coupled by a first pair of sprockets having a first belt trained therebetween and having a first ratio, the first belt and first pair of sprockets comprising a helical tooth configuration, the input shaft and the output shaft coupled by a second pair of sprockets having a second belt trained therebetween and having a second ratio, and the input shaft and the output shaft coupled by a third pair of sprockets having a third belt trained therebetween and having a third ratio.

FIELD OF THE INVENTION

The invention relates to a vehicle steering system transmission, andmore particularly to a vehicle steering system transmission comprisingan input shaft and an output shaft coupled by a first pair of sprocketshaving a ratio, a second pair of sprockets having a ratio and the thirdpair of sprockets having a ratio.

BACKGROUND OF THE INVENTION

Electric power assist steering systems (EPAS) have been around since the1960's. Hydraulic power assist steering has traditionally dominated themarket. Hydraulic systems have high parasitic energy loss when thehydraulic pump is pumping, but power assist is not required. Earlyattempts to eliminate this parasitic loss involved fitting an electricmotor to the pump and only driving the pump when necessary.

Electric hydraulic assisted power steering systems use an electric motorto drive a hydraulic pump to feed a hydraulic power steering system.These systems are an intermediate step by the industry and their usewill likely fade with the increased use of EPAS. EPAS systems allowrealization of reduced noise, reduced energy use, active safetyfeatures, and adjustability to meet driving conditions. However, the useof these systems has remained limited until recent C.A.F.E. requirementsbecame more difficult to meet. This is driving automotive manufacturesto turn to EPAS systems more and more in an effort to improve vehiclefuel economy. EPAS systems eliminate the parasitic losses typicallyfound in hydraulic assist power steering systems. System manufacturerssuch as Nexteer make claims of 6% fuel economy improvements.

For example, one difficulty that slowed implementation of EPAS systemswas meeting the power requirement with a 12 volt electric motor.Recently systems have been developed that successfully solve thisproblem. Further, all EPAS systems require a control module to sensedriver input and control the electric motor to provide the desiredassist. The control module measures driver input torque and uses this todetermine the amount of assist required. Assist can be tuned to meet thedrivers need depending on driving conditions. The system can even have atunable “feel” available to the driver.

Even though the main driver for automotive EPAS is fuel economyimprovement, EPAS has additional benefits. The system can make steeringassist available even when the vehicle's engine is not running. It alsoenables the use of the automatic parallel parking systems availabletoday.

There are two main types of EPAS systems; column assist and rack assist.Rack assist EPAS systems have an electric motor that is connected to thesteering rack. The electric motor assists the rack movement usuallythrough driving a lead screw mechanism. Column assist EPAS systems havean electric motor connected to the steering column. The electric motorassists the movement of the column shaft usually through a worm geartype arrangement. One advantage of these types of systems is theelectric motor can be placed in the passenger compartment freeing upvaluable space under the hood. This also keeps any sensitive electricalcomponents out of the harsh under hood environment.

Worm drive column assist systems are usually used in small cars wherethe assist power requirements are lower than what would be needed in alarge heavy vehicle. These systems are limited by the speed of thesteering wheel and the ratio of the worm drive. The steering wheel atits fastest speed rotates relatively slowly at approximately 60 rpm.With a 60 rpm speed of the steering wheel and a worm drive ratio of15:1, the max speed of the electric motor would only be 900 rpm. Wormdrives are limited to ratios under 20:1 because ratios higher than thatcannot be back-driven.

The steering system must be able to be operated with no power. Thisrequires the worm drive be able to operate with the gear driving theworm (back-driven). Having a low motor speed and limited ratio wormdrive causes the need for high torque motor. Even with a high torquemotor, these types of systems have not been made successful on heavyvehicles. Small vehicles are light and require less steering effort thusenabling the use of these systems. Worm drive column assist EPAS systemsare the lowest cost systems and thus also lend themselves to smallerless expensive vehicles.

Typical steering systems with worm drive assists are limited in theirefficiency. EPAS systems must be designed to operate when there is nopower available. Due to the nature of worm drive's tendency to lock upduring back driving when ratios exceed approximately 20:1, worm driveEPAS systems efficiency is not greater than approximately 85% and nearerto 65% during back-driving conditions.

Today there are no column assist SPAS systems commonly available thatuse anything other than a worm drive to facilitate the assist. Thesecolumn systems are unable to provide enough assist for large heavyvehicles.

Representative of the art is U.S. Pat. No. 7,887,446 which discloses ahelically-toothed-belt transmission device. A backlash “D” isselectively enlarged in a helically-toothed-belt transmission devicethat transmits drive force by meshing between a helically toothed beltand a helically toothed pulley, that is, a tooth helix angle “θ” is setin a range of −0.2≦1−Wxθ/Pt≦0.75, with “Pt” being a tooth pitch, “θ” atooth helix angle, and W the width of the belt. The backlash “D” betweenthe helically toothed belt and the helically toothed pulley is set to be1.6%-3% of the tooth pitch “Pt”.

What is needed is a vehicle steering system transmission comprising aninput shaft and an output shaft coupled by a first pair of sprocketshaving a ratio, a second pair of sprockets having a ratio and the thirdpair of sprockets having a ratio. The present invention meets this need.

SUMMARY OF THE INVENTION

The primary aspect of the invention is to provide a vehicle steeringsystem transmission comprising an input shaft and an output shaftcoupled by a first pair of sprockets having a ratio, a second pair ofsprockets having a ratio and the third pair of sprockets having a ratio.

Other aspects of the invention will be pointed out or made obvious bythe following description of the invention and the accompanyingdrawings.

The invention comprises a vehicle steering system transmissioncomprising a housing, an input shaft journalled to the housing, anelectric motor connected to the housing and coupled to the input shaft,an output shaft journalled to the housing, the input shaft and theoutput shaft coupled by a first pair of sprockets having a first belttrained therebetween and having a first ratio, the first belt and firstpair of sprockets comprising a helical tooth configuration, the inputshaft and the output shaft coupled by a second pair of sprockets havinga second belt trained therebetween and having a second ratio, and theinput shaft and the output shaft coupled by a third pair of sprocketshaving a third belt trained therebetween and having a third ratio.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthe specification, illustrate preferred embodiments of the presentinvention, and together with a description, serve to explain theprinciples of the invention.

FIG. 1 is a cross sectional view of the inventive transmission.

FIG. 2 is a perspective view of the inventive transmission.

FIG. 3 is an exploded view of the inventive transmission.

FIG. 4 is a graph of the efficiency of the transmission as a speedreducer.

FIG. 5 is a graph if the efficiency of the transmission as a speedmultiplier.

FIG. 6 is a perspective view of a prior art electric power assist racksystem.

FIG. 7 is a detail of FIG. 6.

FIG. 8 is a schematic of a steering system.

FIG. 9 schematically shows an arrangement of an endlesshelically-toothed belt installed on a helically-toothed pulley, which isviewed from a back side of the belt.

FIG. 10 is a schematic enlarged view showing the relations between theteeth traces of the helically-toothed pulley and the teeth traces of thehelically-toothed belt engaged thereto.

FIG. 11 illustrates half tooth profiles of the belt and pulley.

FIG. 12 illustrates an angle of the helical tooth applied to the endlessbelt.

FIG. 13 illustrates the helically-toothed belt.

FIG. 14 shows a form of a compressible tooth profile.

FIG. 15 is a detail of the belt arrangement.

FIG. 16 is a perspective view of the inventive transmission in asteering system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a cross sectional view of the inventive transmission. Theinventive transmission 1000 comprises a housing 100. Contained withinthe housing is an input shaft 200. The input shaft is coupled to anelectric motor 201. The electric motor is a 12 V DC motor known in theart. The electric motor is attached to the housing at a motor mount 101.

Input shaft 200 is journalled to the housing by a first bearing 201 anda second bearing 202.

Sprocket 206 is press fit to shaft 200. Sprocket 206 comprises a toothedsurface 207 for engaging a toothed belt 400.

An intermediate sprocket 203 is journalled to shaft 200 by a bearing 208and a needle bearing 205. Sprocket 203 freely rotates on shaft 200.Intermediate sprocket 203 is connected to sprocket 204, in other wordsthey are a single unit. Intermediate sprocket 203 comprises a toothedsurface 209, and sprocket 204 comprises a toothed surface 210, each forengaging a toothed belt. The diameter of toothed surface 209 is greaterthan a diameter of toothed surface 210. The diameter of sprocket 206 isless than the diameter of sprocket 204. Of course, any combination ofdiameters is possible in order to achieve the desired ratio.

An output shaft 300 is journalled to the housing by a first bearing 301and a second bearing 302.

Sprocket 306 is journalled to output shaft 300 by a bearing 308 and aneedle bearing 305. Sprocket 306 comprises a toothed surface 307 forengaging a toothed belt. Sprocket 306 is connected to sprocket 309 whichalso comprises a surface for engaging a toothed belt. The diameter ofsprocket 306 is greater than the diameter of sprocket 309. The diameterof sprocket 306 is less than the diameter of sprocket 303. Of course,any combination of diameters is possible in order to achieve the desiredratio for each pair of sprockets.

Sprocket 303 is press fit to shaft 300. Sprocket 303 comprises a toothedsurface 310 for engaging a toothed belt.

A toothed belt 400 is trained between the first pair of sprockets,namely, sprocket 206 and sprocket 306. The ratio between sprocket 306and 206 is 3.4:1. Toothed belt 400 comprises a helical belt which ismore fully described elsewhere in this specification.

A toothed belt 500 is trained between the second pair of sprockets,namely, sprocket 203 and sprocket 309. The ratio between sprocket 203and 309 is 3.0:1. A toothed belt 600 is trained between the third pairof sprockets, namely, sprocket 204 and sprocket 303. The ratio betweensprocket 303 and 204 is 3.0:1. Toothed belt 500 and toothed belt 600 donot comprise a helical belt as is the case for toothed belt 400.

The dimensions given in this specification are examples only and are notintended to limit the scope of the inventive transmission.

Helical Belt.

Helical belt 400 and helical sprockets 206, 306 are described next. FIG.9 schematically shows an arrangement of an endless helically-toothedbelt installed on a helically-toothed pulley, which is viewed from aback side of the belt. As shown in the figure, the helically-toothedbelt 400 is entrained around a pair of helically-toothed pulleys 206 and306 which are rotatable about respective axes “L1” and “L2”. Forexample, the helically-toothed pulley 306 is a drive pulley and therotational power of the helically-toothed pulley 306 is transmitted tothe driven pulley 206 via the helically-toothed belt 400. In FIG. 9, thehelically-toothed belt 400 described by a solid line indicates anarrangement of the helically-toothed belt immediately after the beltinstallation. On the other hand, a phantom line indicated by a referencenumber 400′ represents a position of the helically-toothed belt 400after the belt-drive transmission device is driven.

Immediately after the helically-toothed belt 400 is installed on thehelically-toothed pulleys 206 and 306 (before the belt-drivetransmission device is driven), teeth traces of the helically-toothedbelt 400 coincide with teeth traces of the helically-toothed pulleys 206and 306, so that the longitudinal direction of the helically-toothedbelt 400 is made substantially perpendicular to the rotational axes “L1”and “L2” of the helically-toothed pulleys 206 and 306. However, when thehelically-toothed pulley 306 or the drive pulley is driven and the loadbears upon the helically-toothed belt 400, the helically-toothed belt400 skids along the teeth traces of the pulleys, thus a thrust occurs.Namely, when the belt-drive transmission device is driven, thehelically-toothed belt 400 skids on the helically-toothed pulley 206 inthe “A” direction along the rotational axis “L1”, and skids on thehelically-toothed pulley 306 in the “B” direction, which is opposite tothe “A” direction, along the rotational axis “L2”, as shown in FIG. 9.Thereby, the helically-toothed belt 400, represented by the solid line,is moved to the position 10′ which is represented by the phantom line.This type of thrust is prominent when the belt-drive transmission deviceis operated under a heavy load or at a high-speed rotation.

FIG. 10 is a schematic enlarged view showing the relations between theteeth traces of the helically-toothed pulley 306 and the teeth traces ofthe helically-toothed belt 400 engaged thereto, after the belt-drivetransmission device operation is started or after thrust has occurred.As shown in the figure, a tooth trace 411 of the helically-toothed belt400 is inclined against a tooth trace 31 of the helically-toothed pulley306 to the amount of angles where the belt has slanted by the skid, sothat the tooth trace 411 slips out of the tooth trace 31. When a gap isinduced between the tooth trace of the helically-toothed belt 400 andthe tooth trace of the helically-toothed pulley 306, inadequate contactis generated between the pulley and the belt. For example a shoulder (apart connecting a working flank and a tooth tip cylinder) of the pulleycontacts a mating flank of the belt, or the like. Such inadequateengagement generates noise and vibration. Note that, as can be seen fromFIG. 9, the same phenomena are induced on the helically-toothed pulley206.

In order to solve such problems, backlash between the belt and thepulley is selectively enlarged in a first embodiment of the presentinvention, so as to prevent inadequate contact between the pulley teethand the belt teeth and reduce noise and vibration. Referring to FIG. 11,the definition of backlash in the present embodiment will be explained.

FIG. 11 illustrates half tooth profiles of the belt and pulley. A solidcurve “P1” describes the tooth profile of the pulley and a broken curve“P2” describes the tooth profile of the belt. A straight line “B”, adash-dotted line, is the base line on a bottom land of the belt, so thata tooth height “H” is defined by the height from the base line B to thetip of the belt tooth. Further, is an arc “C”, which is also indicatedby a dash-dotted line, is an arc that passes through a point that isdistant from the base line “B” by a half of the distance of the toothheight “H” (½H), and its center coincides with the center of the pulley.Namely, the diameter of the arc “C” is equivalent to a value where thetooth heights “H” are subtracted from the outside diameter of thepulley. The backlash between the belt and the pulley is defined by thedistance “D” between the pulley tooth and the belt tooth at a positionalong the arc “C” (the distance between the intersection of the curve“P1” and the arc “C”, and the intersection of the curve “P2” and the arc“C”).

Next, referring to FIG. 12, an angle of the helical tooth applied to theendless belt of the first embodiment will be explained. FIG. 12 is apart of a schematic development of the helically-toothed belt 400, whichis applied to the first embodiment. The teeth traces of thehelically-toothed belt 400 are represented by the slanted solid lineswhich lie in the lateral direction of the belt. Here, the pitch of thebelt teeth is denoted by “Pt” and the width is denoted by “W”. Further,when denoting an angle (tooth helix angle) between the line in thelateral direction of the belt (or the line perpendicular to the beltlongitudinal direction) and the tooth trace as “θ”, a space “d” betweenthe end of the first tooth engagement and the beginning of theneighboring second tooth engagement are represented by d=Pt−Wxθ, usingthe pitch “Pt”, the belt width “W”, and the tooth helix angle “θ”. Inthe first embodiment, the tooth helix angle “θ” is set so as to satisfy−0.2≦d/Pt=1−Wxθ/Pt≦0.75

In a conventional helically-toothed-belt transmission device, althoughthe backlash “D” is set to about 1.5% with respect to the tooth pitch“Pt”, the backlash “D” of the helically-toothed-belt transmission deviceof the first embodiment, is set in a range of 1.6% to 3% (D/Pt×100) ofthe tooth pitch “Pt”.

Namely, in the helically-toothed-belt transmission device of the firstembodiment, an inadequate contact between the teeth of the pulley andthe belt is prevented, even when thrust is induced on the belt when aheavy load is placed upon the belt during operation, by setting thebacklash “D” widely (wide backlash), such as at 1.6% to 3% of the toothpitch “Pt”. Further, this is particularly effective for the tooth helixangles “θ” that satisfy −0.2≦d/Pt≦0.75. Namely, over a wide range oftooth helix angles “θ” (even for a small angle which is not veryeffective for a compressible tooth profile), the noise and the vibrationcan be reduced. As described above, according to the first embodiment,noise and vibration are effectively reduced for thehelically-toothed-belt transmission device which is driven under a heavyload or at a high-speed rotation.

Next, with reference to FIG. 13 and FIG. 14, a belt-drive transmissiondevice of a second embodiment of the present invention will beexplained. FIG. 13 is part of a schematic development of thehelically-toothed belt 400, which is applied to the second embodiment.The teeth traces of the helically-toothed belt 400 are represented bythe slanted solid lines which lie in the lateral direction of the belt.Further, FIG. 14 shows a form of a compressible tooth profile applied inthe second embodiment.

In the belt-drive transmission belt of the second embodiment, the toothhelix angle “θ” is set in a range of d/Pt=1−W tan θ/Pt≦0. Namely, asshown in FIG. 13, a value of “d” is “0” or negative, so that theengagement of the neighboring second tooth starts before the end of thefirst tooth engagement (or simultaneously with the end of theengagement).

In FIG. 14, a curve “P3” indicated by a solid line represents the toothprofile of the helically-toothed pulleys 206 and 306 of the secondembodiment, and a curve “P4” indicated by a broken line represents thetooth profile of the helically-toothed belt 400 of the secondembodiment. Further, a dash-dotted line “B” represents the base line ofthe belt when the belt is installed. A groove depth “Dp”, a depth fromthe base line “B” to the tooth root cylinder of the pulley, is smallerthan the tooth height “H” of the belt by “h”. Therefore, when thehelically-toothed belt 400 is entrained about the helically-toothedpulleys 206 and 306, and tension is given, the belt teeth are pressedagainst the tooth root cylinder of the pulley and compressed. Thereby,positioning accuracy of each of the belt teeth to the pulley grooves isimproved, so that a cumulative error between the belt teeth and thepulley teeth is reduced, and the inadequate contact between the beltteeth and the pulley teeth is prevented. Note that, in the secondembodiment, the compressibility (h/H×100) of the helically-toothed beltis set within 1.5% to 5%.

As described above, according to the second embodiment, noise andvibration are effectively reduced from the helically-toothed-belttransmission device where the tooth helix angle “θ” is within the rangeof d/Pt=1−Wθ/Pt≦0, and where the device is driven under a heavy load orat a high-speed rotation, by preventing the inadequate contact betweenthe belt teeth and the pulley teeth. Note that, thehelically-toothed-belt transmission device of the second embodiment isparticularly effective around a span resonance frequency.

Steering ratio is the ratio of the number of degrees of steering wheelmovement per one degree of front wheel movement. A 20:1 steering ratiorequires 20 degrees of steering wheel movement to move the front wheelsone degree. Most power steering systems have ratios somewhere between12:1 and 24:1. Ratios of 12:1 are for sports cars. A large pick-up truckmay have a ratio near 24:1.

The inventive system consists of a series of belts arranged on twocommon axis, namely, the input and output shafts. The inventive systemprovides a torque multiplication ratio of 30.6:1 from the electric motorto the output shaft 300. This is accomplished through three stages of3.4:1, 3:1 and 3:1.

The initial stage nearest the electric motor 201 is configured for the3.4:1 ratio. First stage sprockets 206 and 306 utilize a helicalsprockets and a helical belt to minimize noise on this high speed belt.The next two sprocket stages utilize a 5 mm pitch toothed belt. Thesprocket tooth combinations chosen enable the design to maintain thesame center distance for both pitch designs.

FIG. 6 is a perspective view of a prior art electric power assist racksystem. The system typically comprises a steering column (S) and a rack(R). A steering wheel is connected to the steering column by which adriver input is received to steer a vehicle. The rack is also known inthe art as a “rack and pinion” steering system.

Rack and pinions are commonly defined by the number of inches of travelof the rack per revolution of the pinion. The exact required rack ratiois dependent on the steering geometry. Steering ratios of 24:1 and 12:1may have a rack ratio of 1.57:1 and 2.62:1 respectively. The ratio of arack and pinion can be varied across the rack. This is accomplishedthrough altering the profile of the teeth of the rack along the rack.This changes the contact radius with the pinion. Changing the contactradius changes the amount of rack travel per turn of the pinion. Thisratio change is limited to a maximum of 15% across the rack.

FIG. 7 is a detail of FIG. 6. The prior art electric power assiststeering system comprises a belt (B) driven by an electric motor (M).The belt is trained between two sprockets (S1) and (S2) and drives aworm gear rack (WG). As a driver turns the steering wheel a controlmodule (not shown) receives a signal which then energizes the motor (M)accordingly in order to drive the belt. As the belt rotates, sprocket(S2) drives the worm gear rack axially in order to move the vehiclewheels to steer.

The power required to steer the front wheels of a vehicle is a maximumwhen the vehicle is not moving. A heavier vehicle requires more power tosteer as well. The following is an example calculation of the powerrequired to steer front wheels of a stopped vehicle.

It is necessary to describe a geometry terms as applied to steeringsystems. The minimum effective radius arm length (A) is the shortesteffective distance from the turning center (B) to the tie rod (C).Usually this is taken when the wheels are fully turned. King pin offset(D) is the distance from the centerline of the wheel (E) to the turningcenter (B). Tire width (F) is the width of the patch of contact betweenthe tire and the road surface.

For the purpose of illustrating the invention, the following informationis given:

Vehicle weight on front axle 900 Kg Gs Tire width 200 mm B Friction;Tire to road 0.8 μ Min effective radius arm for str. 0.1 m r King pinoffset 100 mm eThe torque M required to steer the wheels can be calculated using thefollowing formula:

$\begin{matrix}{M = {0.05*{Gs}*\frac{1}{\left( {1 + \frac{e}{B}} \right)}*\frac{B}{200}*\frac{\mu}{0.7}}} \\{M = {336\mspace{14mu}{Nm}}}\end{matrix}$

If one assumes the wheel total angular displacement is degrees and ittakes 2 seconds to turn the steering wheel from lock to lock, the powerrequirement can be calculated as follows:

Wheel angular displacement 85 deg Time to turn lock to lock  2 secThen the angular speed of wheels during turning:

$\begin{matrix}{\omega = {\frac{{angular}\mspace{14mu}{{displacement}({rev})}}{{time}\left( {\sec.} \right)}*60\left( \frac{\sec}{\min} \right)}} \\{\omega = {\frac{\left( \frac{85}{360} \right)}{2}*60}} \\{\omega = {7.08\mspace{14mu}{rpm}}}\end{matrix}$Power required without any system losses:

$\begin{matrix}{P = {T*\omega*\left( \frac{2\pi}{60} \right)}} \\{P = {336*7.08*\left( \frac{2\pi}{60} \right)}} \\{P = {249\mspace{14mu}{watt}}}\end{matrix}$If it is assumed the vehicle steering system has an efficiency of 70%,the system has an efficiency of 80%, and the worm gear has an efficiencyof 80%, the power requirement is:

$\begin{matrix}{P = \frac{P}{e}} \\{P = \frac{249}{\left( {0.7*0.8*0.8} \right)}} \\{P = {556\mspace{14mu}{watt}}}\end{matrix}$

On the other hand, the inventive system uses three belt stages. FIG. 4is a graph of the efficiency of the transmission as a speed reducer.FIG. 5 is a graph if the efficiency of the transmission as a speedmultiplier. Using this information as a surrogate for determining theefficiency of the proposed system, it is expected that the inventivesystem would have efficiencies consistently above 95%.

Operation, Electric Motor Assist Mode.

In operation a driver will turn a vehicle steering wheel which isconnected to shaft 300. A typical vehicle system will include a controlmodule to sense driver input and control the electric motor 201 toprovide the desired assist through the transmission 1000. For example,the control module measures driver input torque and uses this todetermine the amount of assist required from the electric motor 201.Assist can be tuned to meet the drivers need depending on drivingconditions.

When assist is being demanded by the control module or ECU, theenergized electric motor will apply torque to shaft 200, this in turnprovides torque to sprocket 206. During operation input shaft 200 mayrotate at up to 1800 RPM. This results in a rotational speed of theoutput shaft of approximately 60 RPM given a reduction through thetransmission of 30.6:1, which represents a typical upper limit. Use ofthe helical belt 400 and helical sprockets 206, 306 significantly quietthe relatively high speed operation of the belts. Straight toothedbelts, such as belt 500 and belt 600, can produce a noise or whine whenoperated and high speeds. The helical tooth design allows for a moregradual meshing between the teeth of the belt and the sprocket groovesduring operation.

The shaft speed is not a significant issue for belt 500 and belt 600since these operate at the maximum speed of 1800/3.4=529 RPM (belt 400)and 529/3.0=176 RPM (belt 500) and 176/3.0=58 RPM (belt 600).

Toothed belt 400 transmits the force from sprocket 206 to sprocket 306,which in turn applies a torque to sprocket 309. Sprocket 309 drivessprocket 203 through belt 500. Sprocket 204 drives sprocket 303 throughbelt 600.

Hence, the torque flow during assist mode is from the electric motor 201to shaft 200 to sprocket 206 to belt 400 to sprocket 306 to sprocket 309to belt 500 to sprocket 203 to sprocket 204 to belt 600 to sprocket 303to shaft 300.

Operation, No Assist Mode.

When no electric assist is required by the control module, a driverinput will apply a torque to shaft 300. Even though shaft 300 rotates,since electric motor 201 is de-energized it will rotate freely and thesystem will operate as though no electric motor is present.

FIG. 2 is a perspective view of the inventive transmission. Electricmotor 201 is mounted to housing 100. Housing 100 encases the sprocketsand belts to protect them from debris.

FIG. 3 is an exploded view of the inventive transmission. The inventivesystem is relatively compact. The three belt stages are contained in asingle housing 100. The housing is sufficiently compact to allowinstallation in a vehicle steering system. Depending upon torquerequirements, the width of each belt may be increased or decreased aswell.

The inventive system is completely scalable. Based on calculations ofsmall car system steering efforts, the worm drive systems provideapproximately 80% of the torque needed to steer a stopped vehicle.Existing worm systems provide approximately 30 Nm of assist. Theinventive system is designed to provide assist for heavy pick-up trucktype vehicles requiring 70 Nm of assist. In order to provide the samelevel of torque assist as the existing worm drive systems, the beltwidths could be narrowed to optimize their design width and the motortorque requirement could be reduced to account for the additionalmechanical advantage of the 30.6:1 ratio for the inventive transmission.It is expected the inventive transmission could provide up toapproximately 150 Nm of assist.

The belts can also be made wider to provide greater assist forapplications such as heavy trucks and buses. It is estimated that alarge pickup truck requires approximately 90 Nm to turn the wheels on astopped vehicle which translates to approximately 70 Nm of assistrequired.

FIG. 15 is a detail of the belt arrangement. A toothed belt 400 istrained between the first pair of sprockets, namely, sprocket 206 andsprocket 306. A toothed belt 500 is trained between the second pair ofsprockets, namely, sprocket 203 and sprocket 309. A toothed belt 600 istrained between the third pair of sprockets, namely, sprocket 204 andsprocket 303.

FIG. 16 is a perspective view of the inventive transmission in asteering system. A steering column (S) is connected to one end of shaft300. The other end of shaft 300 is connected to an input portion of asteering rack (R). Steering rack (R) is known in the art and istypically included in a system known as a “rack and pinion” steeringsystem. Steering column (S) is typically connected to a steering wheelby which a driver steers the vehicle.

Although a form of the invention has been described herein, it will beobvious to those skilled in the art that variations may be made in theconstruction and relation of parts without departing from the spirit andscope of the invention described herein.

1. A vehicle steering system transmission comprising: a housing; aninput shaft journalled to the housing; an electric motor connected tothe housing and coupled to the input shaft; an output shaft journalledto the housing; the input shaft and the output shaft coupled by a firstpair of sprockets having a first belt trained therebetween and having afirst ratio, the first belt and first pair of sprockets comprising ahelical tooth configuration; the input shaft and the output shaftcoupled by a second pair of sprockets having a second belt trainedtherebetween and having a second ratio; and the input shaft and theoutput shaft coupled by a third pair of sprockets having a third belttrained therebetween and having a third ratio.
 2. The vehicle steeringsystem transmission as in claim 1, wherein the second belt and the thirdbelt are toothed.
 3. The vehicle steering system transmission as inclaim 1, wherein the first ratio is 3.4:1.
 4. The vehicle steeringsystem transmission as in claim 3, wherein the second ratio is 3:1. 5.The vehicle steering system transmission as in claim 3, wherein thethird ratio is 3:1.
 6. The vehicle steering system transmission as inclaim 1, wherein the electric motor comprises a 12 V DC motor.
 7. Avehicle steering system transmission comprising: a housing; an inputshaft journalled to the housing; a steering column coupled to an outputshaft; the output shaft coupled to a steering rack; an electric motorconnected to the housing and coupled to the input shaft; the outputshaft journalled to the housing; the input shaft and the output shaftcoupled by a first pair of sprockets having a first belt trainedtherebetween and having a first ratio; the input shaft and the outputshaft coupled by a second pair of sprockets having a second belt trainedtherebetween and having a second ratio; and the input shaft and theoutput shaft coupled by a third pair of sprockets having a third belttrained therebetween and having a third ratio.
 8. The vehicle steeringsystem transmission as in claim 7, wherein the first belt and first pairof sprockets comprise a helical tooth configuration.
 9. The vehiclesteering system transmission as in claim 7, wherein the second belt andthe third belt are toothed.
 10. The vehicle steering system transmissionas in claim 7, wherein the first ratio is 3.4:1.
 11. The vehiclesteering system transmission as in claim 10, wherein the second ratio is3:1.
 12. The vehicle steering system transmission as in claim 10,wherein the third ratio is 3:1.
 13. A vehicle steering systemtransmission comprising: a housing; an input shaft journalled to thehousing; an electric motor connected to the housing and coupled to theinput shaft; an output shaft journalled to the housing; the input shaftand the output shaft coupled by a first pair of sprockets having a firstbelt trained therebetween and having a ratio of 3.4:1, the first beltand first pair of sprockets comprising a helical tooth configuration;the input shaft and the output shaft coupled by a second pair ofsprockets having a second belt trained therebetween and having a ratioof 3:1; and the input shaft and the output shaft coupled by a third pairof sprockets having a third belt trained therebetween and having a ratioof 3:1.
 14. The vehicle steering system transmission as in claim 13,wherein the electric motor comprises a 12 V DC motor.